In order to meet the demand for smaller semiconductor IC packages with more pins, the BGA type semiconductor IC package has come into the spotlight as a replacement of the conventional QFP (Quad Flat Package) and TCP (Tape Carrier Package). The BGA (Ball Grid Array) type semiconductor IC is a surface mount type package in which the solder bumps as the connecting terminals are arranged two-dimensionally on the bottom surface of the insulating substrate on which the IC chip is assembled. As the connecting terminals are arranged in a two-dimensional pattern, even when the number of pins is increased, the dimensions of the package still do not become larger, and the terminal pitch can be selected as 1 mm or larger, so that the conventional overall reflow assembly technology can be adopted. This is an advantage of BGA.
One of the BGA type semiconductor IC packages has solder bumps connected to a conductor pattern formed as a prescribed pattern on the surface on the opposite side (the side of the IC chip) of the insulating substrate with through-holes formed two-dimensionally on the insulating substrate. The following method is usually adopted: in the package of this constitution, the through-holes are filled with solder paste by means of a squeezer; nearly spherical solder balls are arranged above them, and they are melted by means of overall reflow. The solder paste filling the through-holes acts as a bridge between the conductor pattern and the solder balls. If the solder paste were not there, it would be difficult for the solder to fill the through-holes completely during the reflow operation, and gaps would form between the conductor pattern and the solder balls, so that bonding to the conductor pattern would not be realized. This is because the solder balls are acted upon by a force that tends to maintain its spherical form, that is, surface tension.
On the other hand, with respect to the necessity of the solder paste, presence/absence of the step of operation of filling the solder paste into the through-holes has a major influence on the productivity and cost of the package.
Basically, the distance between the conductor pattern and the solder balls depends on the thickness of the insulating substrate, the diameter of the through-holes and the diameter of the solder balls. Consequently, when the solder balls are to fill the through-holes completely without using the solder paste, the diameter of the through-holes must be further increased or the diameter of the solder balls decreased.
However, when the diameter of the through-holes is increased while the pitch between the through-holes is kept unchanged, the line width of the several lines of the conductor pattern passing through and between the through-holes must be reduced, or the interlinear distance must be made smaller. This is a disadvantage. Consequently, this should be avoided as much as possible so as to prevent a decrease in the impedance of the conductor pattern or crosstalk between [portions] of the conductor pattern.
Also, the diameter of the solder balls cannot be too small or the connection reliability with respect to the mounting substrate cannot be maintained. In other words, the diameter of the solder balls must be larger than a prescribed value in consideration of the warping of the package and the mounting substrate during package assembly, and the package height after assembly.
The purpose of the present invention is to provide a type of IC package which has solder bumps bonded to the conductor pattern on the opposite side of the insulating substrate without filling the solder paste in the through-holes. The present invention also provides a manufacturing method for the aforementioned IC package.
Another purpose of the present invention is to provide a type of bump structure and its formation method in which the processing step of filling the solder paste is omitted without changing the thickness of the insulating substrate, diameter of the through-holes, diameter of the solder balls, and other basic dimensions, so as to improve the IC package fabrication efficiency.
On the other hand, when the IC package is mounted on an external substrate, thermal stresses occur at the solder bonding portion due to the differences in the thermal expansion coefficients of the mounting substrate and the package. Thermal stresses cause cracks to form in the solder bonding portion and lead to poor assembly of the package. Consequently, to improve the assembly reliability of the package, it is extremely important to minimize the thermal stresses at the solder bonding portion. Particularly for the BGA type semiconductor IC package, not only must the thermal stresses at the bonding portion between the solder balls and the mounting substrate be taken into consideration, but also the thermal stresses that take place at the bonding portion between the solder balls and the package-side conductor pattern.
Another purpose of the present invention is to improve the assembly reliability of the BGA type semiconductor IC package by reducing the thermal stresses that take place at the bonding portion between the solder balls and the package-side conductor pattern.